As I have pointed out, these cars do just fine when warm without the electronic correction.
There are two possibilities to what you are seeing. Just last week we had a car come in complaining of poor cold starting. A quick look to check EHA current (ma not mv) showed that the connector was off. Easy cold start repair, but, plugged in the car ran worse and worse. While monitoring, the EHA current would slowly drop (once the connection was made) till the lambda correction was all used up. It went to -12ma. This means the controller thinks the car is running rich and it subtracts fuel (lean correction). So, next we disconnected the O2 sensor hoping that the sensor was giving a false reading (it would have had to see a voltage on the sensor as a ground would have sent the system the other way to +12ma). For those ready to test, the EHA ma will go to +12ma if one grounds the O2 lead to the controller. If one places one hand on 12v and the other on the sensor it will see enough voltage to go the other way, as we saw it, at -12ma. This test is a very good way to see if the system has full corrective capability. Depending on initial calibration the system usually goes far enough in both directions to run poorer with an especially good area somewhere in the middle.
It turned out that our car had a bad controller (car was a recent purchase) and someone had disconnected the EHA to solve the lean correction.
That possibility is very rare. Controllers seldom fail. It's possible for the heater circuit of the O2 sensor to bleed enough voltage into the O2 sensor signal for similar to happen. A short will make it rich and that usually makes it run better not worse.
The other possiblity is that the car doesn't like running at the lambda control mixture. This is common but usually is only an idle thing.
You need to get that meter back hooked up. Remember that you are checking current. For current to flow it will pass through the meter. When in the mv scale the current will not pass through the instrument so the circuit is not complete and the car will run as if disconnected. To verify your hook up turn the key on with engine off. When warm you should see about 18-20ma. During cranking it willl jump by 30-50ma. With the O2 sensor disconnected (under carpet passenger side), you should get 0ma when warm. If you drive with the meter hooked up the reading will jump +6-10ma with a sudden acceleration. While at constant speed the reading should fluctuate up and down about 2-3ma, centered about zero or a little negative (I usually set them to -2ma to zero ma).
Bosch Master, ASE Master, L1
33 years MB technician